From sci.eletronics.repair group, useful to know tidbit:
Mark Storkamp Oct 9, 8:47 am
- Show quoted text -
For any size thread you plan to tap, to find the hole size just subtract
the pitch of the thread from the diameter of the thread. For an M3x.5
that would be 2.5, for a 1/4-20 it would be .25-1/20, .25-.05, or .2 (#7
drill is .201), etc.
also I have four pages of tables
metric-fine
<http://www.coasteltools.com/tech_tap_drill_mf.htm
metric-coarse
<http://www.coasteltools.com/tech_tap_drill_mc.htm
National-coarse
<http://www.coasteltools.com/tech_tap_drill_nc.htm
National-fine
<http://www.coasteltools.com/tech_tap_drill_nf.htm
don't know if they still exist, though

That's a nice link. It includes the chart I linked to as well as a few
other useful charts.
Here's another chart that I have hanging next to my drill press. It comes
in handy every now and then...
http://images.meredith.com/wood/pdf/drill-press-speed-chart.pdf

On Sun, 19 Jan 2014 06:08:32 -0800 (PST), herb white
Lots of them online - or do the simple calculation - nominal diameter
less .75Xpitch is a good rule of thumb. so a 1/4X20 gets ..2125 - the
tables say.2188 for 50% thread
1/2X20 would be .4625 - tables say .4688 for 50% thread (iron and
steel). For plastic or brass the book says .4531 (75% thread).
BUT - you obviously have a computer with internst connection so just
google "tap drill size" and print off the chart.

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